Fast neutron imaging is a technique in which neutrons from a point source penetrate through a sample under examination and show a shadow image when detected by a position sensitive detector. While the technique is similar in principle to some aspects of X-ray or gamma radiography, the cross sections of neutron interaction with nuclei differ significantly from those of X-ray or gamma interactions. Hence, it is possible to use fast neutrons to obtain images of light elements including, for example, a hydrogen rich material beneath a shield of lead or tungsten. Accordingly, fast neutron imaging is of considerable interest for non-destructive testing including material testing, medical and biological applications and cargo screening.
However, while the technique of fast neutron imaging was known prior to the present invention, the methods typically described in the literature involve integrating the detector response. In such methods many events of neutron interaction in a given small area of the detector produce an integrated local response that is displayed as a single piece of information after the exposure is complete. Individual events of neutron interactions are not recorded and therefore the images obtained cannot distinguish between neutrons of different energies and cannot separate backgrounds very well.
More specifically in the known methods of neutron imaging, detection is accomplished by means such as photographic film, image plates, amorphous silicon flat plate detectors, or CCD or similar electronic camera based systems with a neutron sensitive scintillator or position-sensitive photomultiplier tubes with optical fiber optic readout. Common shortcomings of these methods include the lack of neutron energy resolution or even the capability to set up a neutron energy threshold, and the impossibility to separate the neutron signal from such background noise contributions as x-ray or gamma radiation fields and/or thermal neutrons.
Prior to the present invention attempts to achieve event-by-event fast neutron imaging detection have included the use of gas field detectors and conventional photomultiplier tubes with plastic scintillators. These have manifested significant deficiencies with the former having low efficiency and the latter having limited position resolution.